The present invention relates to a friction stielding method and a friction stir welding apparatus; and, in particular, the present invention to a friction stir welding method and a friction stir welding apparatus to be used, for example, in welding an aluminum alloy frame member etc. for use in a side body structure of a car body in a railway vehicle.
A friction stir welding method is a method in which a round rod (referred to hereinafter as a rotary body) is rotated and inserted in a welding portion to be subjected to welding, and then the rotating body is moved along a welding line. The material which exists in the welding portion is exothermally heated and softened and is further elastically fluidized by the rotating body, and the fluidized material effects a solid phase welding according to the friction stir welding method.
The rotary body used in the friction stir welding method and in the apparatus for carrying out friction stir welding comprises a small diameter portion which is inserted fully into the welding portion of the members to be subjected to welding and a large diameter portion which is positioned adjacent the small diameter portion, and this large diameter portion of the rotary body is also inserted to a small extent into the welding portion. The small diameter portion and the large diameter portion of the rotary body are arranged integrally on the same axis, so that both the small diameter portion and the large diameter portion of the rotary body are rotated together. A boundary face portion between the small diameter portion and the large diameter portion of the rotary body is inserted also to a small extent into the welding portion to be subjected to welding.
In general, a gap is formed between side vertical wall portions of two welding members to be welded. The material which exists in the welding portion to be subjected to welding in the gap is blown off and the material surrounding the gap becomes thin. Accordingly, an end portion of the welding member to be welded is formed with an increased thickness portion, namely, the welding member to be welded has an increased thickness portion at the end thereof. This friction stir welding technique, as applied to two members to be welded having an increased thickness portion at the end thereof, is disclosed in Japanese application announcement publication No. Hei 7-505090 (EP 0615480 B1), "Welding & Metal Fabrication" January 1995, pages 13, 14 and 16, and EP 0797043 A2.
Further, the material of the member to be welded is described in the above stated Japanese application announcement publication No. Hei 7-505090. As to the material to be subjected to welding, the material, size, rotation speed, and moving speed of the rotary body is disclosed in "Study of Friction Stir Welding" at Welding Association Japan Meeting Symposium Summary: No. 56 ('95-4), pages 208 and 209.
The friction stir welding technique is one welding method in which the material of the member to be subjected to welding is softened and welded, but there is no supplement of the metal in this process. Accordingly, in the case where two abutting members are welded, when in the area of the gap formed between the two members, the thickness of a welding portion will become thin. In the above stated EP 0797043 A2, providing an increased thickness of the members to be welded in the welding portion to be subjected to welding, the metal from the increased thickness portion is used to supplement the supply of metal to the gap during the welding process.
However, when the size (horizontal width) of the gap formed between the two members to be welded is larger than the diameter of the small diameter portion of the rotary body, it is impossible to properly carry out the friction stir welding. Even when the size of the gap is smaller than the diameter of the small diameter portion of the rotary body, the friction stir welding will become insufficient if the size of the gap is nearly equal to the diameter of the small diameter portion of the rotary body.
The size of the gap is determined easily by the manufacturing accuracy of the members to be welded. In particular, for example, in a case where a side body structure of a car body of a railway vehicle is manufactured by welding aluminum alloy along the longitudinal length of extruded frame members, when an attempt is made, to weld the long frame members, since the size of the gap may be large, the friction stir welding becomes difficult. When the member to be welded is long in length, since the manufacturing accuracy of the welding portion of the members to be welded becomes significant, the management of the size of the gap becomes difficult, with a result that the friction stir welding technique becomes difficult.
Further, the problem of gap size is effected by the accuracy of fixing the members to be welded to a frame stand. In this regard, the longitudinal length of the extruded frame member for the side body structure of the car body of the railway vehicle is about from 20 m to 25 m.